My GPS loses its
signal whenever I run my radar. It’s mounted about three feet behind
the open-array radar antenna on the flying-bridge hardtop. I raised it
on a two-foot extension to no avail.D.D., via e-mail

The NMEA Installation
Standard states that GPS antennas “shall not be installed in the
direct beam of a radar antenna.” That beam is generally specified
as 25 degrees vertically—12 1/2 degrees below the horizon, 12 1/2
degrees above—so your newly raised antenna should be working. However,
the beam edge is also defined as where the radar signal is less than half
of its nominal strength; apparently even a small percentage of your fairly
powerful radar is able to smother your GPS antenna from a few feet away.
Try moving it again, and bear in mind that since it is trying to hear
signals from the sky, not from over the horizon, it will likely be quite
content down on the side of your flying bridge or even on your trunk cabin
top, lower but well away from your other antennas. That’s why most
current GPS antennas can be alternatively mounted directly, and with low
profile, onto a horizontal surface. But I’ve also heard of boaters
who’ve drilled a hole in a metal pie plate and stuck it under a mast-mounted
GPS antenna to protect it from the sort of low-angle interference you
are experiencing—though it strikes me as a stubborn approach!

The whole business of
interference between various active and passive marine electronics is
complicated. For instance, GPS satellite signals are so weak that they
can get messed up by nearby VHF, SSB, satcom, and even cellphone transmissions.
It’s not for nothing that the NMEA Standard has both a table of suggested
minimum horizontal spacing for all antennas and a rigorous protocol for
testing each new installation for interference to or from each existing
installation. —B.E.